Building Permint's Access Commerce Platform

Challenge

Studios, organizers and service businesses sell limited capacity — classes, events, multi-visit passes — and want to be paid directly. Every pass has to be impossible to forge and redeemable exactly once, at the door, on a phone, even when many people buy the last spots at the same moment.

Approach

• Merchants get paid directly. Stripe Connect charges go to each merchant's own account, and an offering can't be published until that account can accept payments.
• Reserve before you charge. Inventory is reserved under row locks before checkout begins, the server sets the price, and the checkout session expires with the reservation.
• Fulfil only from verified webhooks. Signed webhooks are processed exactly once through an event ledger in a single transaction, and a reconciliation job recovers anything missed.
• Passes are cryptographically signed. Each pass is a signed entitlement; redemption is atomic, idempotent and scoped to the merchant. Refunds revoke passes, and disputes suspend them until resolved.

Implementation

• Checkout, purchase-lifecycle and reconciliation services, plus a transactional email outbox with retries.
• Strict per-merchant data isolation, CSRF/CSP/HSTS protections, rate limits and production secret enforcement.
• A dependency-free embeddable widget so merchants can sell from their own websites.
• A SwiftUI iOS app for organizers: dashboard, offerings, sales and QR validation that verifies pass signatures.

Outcome

In documented concurrency tests, twelve simultaneous checkouts against a capacity of three produced exactly three sales, ten simultaneous redemptions of one pass produced exactly one, and duplicate webhooks fulfilled exactly once. Permint is preparing for public launch.

Technical perspective

Almost every hard problem here is about concurrency and retries: reservations, webhook duplication and ordering, simultaneous redemptions. The same guarantees then have to hold across the web dashboard, the embedded widget and the mobile app.